The invention relates to methods for making metallic alloys such as titanium base alloys into castings of various symmetric and asymmetric shapes, cylinders, hollow tubes, pipes, rings and other tubular products by melting the alloys in a vacuum or under a low partial pressure of inert gas and subsequently centrifugally casting the melt under vacuum or under a low pressure of inert gas in molds machined from fine grained high density, high strength isotropic graphite, the said molds either revolving around its own horizontal or vertical axis or centrifuging around a vertical axis of rotation.
The combination of high strength-to-weight ratio, excellent mechanical properties, and corrosion resistance makes titanium the best material for many applications. Titanium alloys are used for static and rotating gas turbine engine components. Some of the most critical and highly stressed civilian and military airframe parts are made of these alloys. The use of titanium has expanded in recent years from applications in aerospace structure to food processing plants and from oil refinery heat exchangers to marine components and medical prostheses. However, the high cost of fabricating titanium alloy components may limit their widespread use.
Some materials which have been found to give excellent results in certain areas of application are listed below by way of example: Pure Ti, Ti-6Al-4V, Ti-6Al-2Sn-4Zr-2Mo, Ti-5Al-2.5Fe, Ti-15V-3Al-3Cr-3Sn, Ti-46Al-2Cr-2Nb, Ti-50Al.
Another family of titanium alloys based on the intermetallic Ti-50Al compositions are being considered for various applications because of their low density, relatively high strength at high temperatures, and corrosion resistance.
While complex shapes of titanium alloys are fabricated by the casting route, somewhat simpler shapes such as seamless rings, hollow tubes and pipes are manufactured by various other thermo-mechanical processing routes. The relatively high cost of titanium components is often fabricating costs, and, usually most importantly, the metal removal costs incurred in obtaining the desired end-shape. As titanium has become a commonly used engineering material there has been a need to produce complex shapes economically. As a result, in recent years a substantial effort has been focused on the development of net shape or near-net shape technologies such as powder metallurgy (PM), superplastic forming (SPF), precision forging, and precision casting. Precision casting is by far the most fully developed and the most widely used net shape technology.
High performance titanium castings are used in large numbers in the aerospace industry while the chemical and energy industries primarily use large castings where corrosion resistance is a major consideration in design and material choice. The microstructure of as-cast titanium is desirable for many mechanical properties such as creep resistance, fatigue crack growth resistance, fracture resistance and tensile strength. Titanium castings are essentially equal in strength, fracture toughness and fatigue crack growth resistance to the corresponding wrought products.
Many titanium castings with precision and complex geometries are made by the well known investment casting process wherein an appropriate melt is cast into a preheated ceramic investment mold formed by the lost wax process, the castings are generally made in static molds. Although defects such as inclusions, gas porosity, hot tears, shrink cavities and mold/metal reactions are common to all foundry products, dealing with these problems require a different approach when casting titanium. The inability to superheat titanium melt in a cold crucible coupled with narrow liquidus /solidus temperature of molten titanium often requires the need of the centrifugal casting technique for making high quality thin walled configurations. A typical centrifugal investment casting machine spins radially symmetric molds about its own axis in a vertical orientation. Simultaneous rotation of a tree of molds located along the perimeter of a circle on a horizontal plane where melt is poured into a central sprue lying along the vertical axis of the tree creates high velocity flow of titanium melt under the action of centrifugal force. By rotation of the tree the melt flows into the mold cavities, keeping contact with one of the vertical inside walls of a gate and a mold cavity. Centrifugal force allows the melt to flow into even the most obscure crevices of the mold cavities The action of centrifugal force leads to improved mold filling and production of high quality precision castings of titanium alloys. The centrifugal force imposed on the melt enhances removal of gas bubbles and reduces the number of gaseous defects to a minimum and improves the mechanical properties.
U.S. Pat. No. 6,250,366, U.S. Pat. No. 6,408,929 and U.S. Pat. No. 6,443,212 disclose a technique and apparatus suitable of production of titanium castings via centrifugal casting in which the molds are arranged about a central axis of rotation like the spoke of a wheel, thus permitting multiple castings is also used to produce sound titanium castings. However, there are certain drawbacks associated with centrifugal casting of titanium in ceramic investment molds. During high velocity flow of melt through the mold cavities under the action of centrifugal force, ceramic walls/linings of the molds in contact with the highly reactive titanium base alloy melts are likely to cause cracking and spalling leading to formation of very rough, outside surface of the casting. The ceramic liners spalling off the mold are likely to get trapped inside the solidified titanium castings as detrimental inclusions which will significantly lower fracture toughness properties of the finished products.
Titanium alloys are fabricated in shapes such as seamless ring configurations, hollow tubes and pipes and find many engineering applications in jet engines such as compressor casings, seal and other high performance components for oil and chemical industries. FIG. 1 shows a diagram of a turbine casing 10 and a compressor casing 20. The compressor casing is made of titanium alloys. FIG. 2 shows a cutway diagram of a turbofan engine and the compressor casing 30 made of titanium alloy. Seamless rings can be flat (like a washer), or they can feature higher vertical walls (approximating a hollow cylindrical section). Heights of rolled rings range from less than an inch up to more than 9 ft. Depending on the equipment utilized, wall-thickness/height ratios of rings typically range from 1:16 up to 16:1, although greater proportions have been achieved with special processing.
There are two primary processes for fabricating seamless rings of titanium alloys. In the ring forging process also called saddle-mandrel forging, an upset and punched ring blank is positioned over a mandrel, supported at its ends by saddles on a forging press. As the ring is rotated between each stroke, the press ram or upper die deforms the metal ring against the expanding mandrel, reducing the wall thickness and increasing the ring diameter.
In continuous ring rolling, seamless rings are produced by reducing the thickness of a pierced blank between a driven roll and an idling roll in specially designed equipment. Additional rolls (radial and axial) control the height and impart special contours to the cross- section. Ring rollers are well suited for, but not limited to, production of larger quantities, as well as contoured rings. In practice, ring rollers produce seamless rolled rings to closer tolerances or closer to finish dimensions. FIGS. 3A-3G schematically show the various steps of seamless rolled ring forging process operations. FIG. 4 shows a ring rolling machine in operation.
FIGS. 3A-3G show an embodiment of a seamless rolled ring forging process operation to make a ring 40. FIG. 3A shows the ring rolling process typically begins with upsetting of the starting stock 42 on flat dies 44 at its plastic deformation temperaturexe2x80x94in the case of grade 1020 steel, approximately 2200 degrees Fahrenheit, to make a relatively flatter stock 43. FIG. 3B shows that piercing the relatively flatter stock 43 involves forcing a punch 45 into the hot upset stock causing metal to be displaced radially, as shown by the illustration. FIG. 3C shows a subsequent operation, namely shearing with a shear punch 46, serves to remove a small punch out 43A to produce an annular stock 47. FIG. 3D shows removing the small punch out 43A produces a completed hole through the annular stock 47, which is now ready for the ring rolling operation itself At this point the annular stock 47 is called a preform 47. FIG. 3E shows the doughnut-shaped preform 47 is slipped over the ID (inner diameter) roll 48 shown from an xe2x80x9cabovexe2x80x9d view. FIG. 3F shows a side view of the ring mill and preform 47 workpiece, which squeezes it against the OD (outer diameter) roll 49 that imparts rotary action. FIG. 3G shows this rotary action results in a thinning of the section and corresponding increase in the diameter of the ring 40. Once off the ring mill, the ring 40 is then ready for secondary operations such as close tolerance sizing, parting, heat treatment and test/inspection.
FIG. 4 shows a photograph of a ring 40 roll forging in operation.
Rings featuring complex, functional cross-sections are produced by machining or forging of simple rings. Aptly named, these xe2x80x9ccontouredxe2x80x9d rolled rings can be produced in many different shapes with contours on the inside and/or outside diameters.
Production of titanium alloy rings from forging billets requires multiple steps by ring rolling. These alloys are difficult to hot work and can be hot deformed with small percentage of deformation in each step of ring roll forging. After each deformation operation, the outside and inside diameters of the stretched ring need to be ground to remove oxidized layers and forging cracks before reheating the ring for the next cycle of hot forging. Because of the extensive fabrication steps involved, the production costs are very high and yields are low. Typically, a 60 inch diameter ring weighing 250 lbs. suitable for application as a large jet engine casing is produced by ring roll forging of a starting billet weighing 2000 lbs. The high loss of expensive materials during fabrication steps results in high cost of the finished products.
A viable alternative to the conventional ring rolling process for fabricating seamless rings, contoured rings and other tubular shapes is horizontal centrifugal casting also known as true centrifugal casting which spins the mold around its own axis. Castings produced by this technique will always have a true cylindrical bore or inside diameter regardless of shape or configuration. Castings produced by this method undergo directional cooling or solidification from the outside of the casting towards the axis of rotation. The mechanical properties of centrifugally cast tubes are often equivalent to conventionally cast and hot-worked material. The uniformity and density of centrifugal castings approaches that of wrought material, with the added advantage that the mechanical properties are nearly equal in all directions. Many engineering ferrous and non-ferrous alloys which are amenable to processing by air melting and casting can be conveniently processed in tubes by centrifugal casting in air. However, reactive titanium alloys require melting and casting in vacuum. Furthermore, during high speed rotation of the centrifugal mold lined with high purity ceramics, the highly reactive titanium base alloy melts are likely to cause cracking and spalling of the ceramic liner leading to formation of very rough, outside surface of the cast tube. The ceramic liners spalling off the mold are likely to get trapped inside the solidified superalloy tube as detrimental inclusions which will significantly lower fracture toughness properties of the finished products.
Casting of titanium and titanium alloys requires special melting, mold-making practices, and equipment to prevent alloy contamination. Because of highly reactive characteristics of titanium with ceramic materials, expensive mold materials (yttria, thoria and zirconia) are used to make investment molds for titanium castings. At elevated temperatures, titanium and its alloys react with the mold facecoat that typically comprises a ceramic oxide to form a brittle, oxygen-enriched surface layer, known as alpha case, which adversely affects mechanical properties of the casting. Alpha case produced in commercial titanium casting processes may range from about 0.005 inches to 0.04 inches in thickness depending on process and casting size. It is removed by a post-casting chemical milling operation as described, for example, in Lassow et al. U.S. Pat. No. 4,703,806. Strict EPA regulations have to be followed to pursue chemical milling. Moreover, ceramic oxide particles originating from the mold facecoat can become incorporated in the casting below the alpha case layer as sub-surface inclusions by virtue of interaction between the reactive melt and the mold facecoat as well as mechanical spallation of the mold facecoat during the casting operation. The sub-surface oxide inclusions are not visible upon visual inspection of the casting, even after chemical milling. However, any sub-surface ceramic inclusions located below the alpha case in the casting are not removed by the chemical milling operation and can lead to degradation of mechanical properties. The extra cost imposed by the chemical milling operation is a disadvantage and presents a serious problem from the standpoint of accuracy of dimensions. Normally, the tooling must take into consideration the chemical milling which results in the removal of some of the material to produce a casting that is dimensionally correct. However, because casting conditions vary, the alpha case will vary along the surface of the casting. This means there is a considerable problem with regard to dimensional variation.
Feagin, U.S. Pat. No. 5,630,465 discloses ceramic shell molds made from yttria slurries, for casting reactive metals. Richerson, U.S. Pat. No. 4,040,845 shows a ceramic composition for crucibles and molds containing a major amount of yttrium oxide and a minor amount of a heavy rare earth mixed oxide. Such methods including the making of a titanium metal enriched yttrium oxide were only partially successful because of the elaborate and expensive technique which required repetitive steps. Schneider, U.S. Pat. No. 3,815,658 shows molds which are less reactive to steels and steel alloys containing high chromium, titanium and aluminum contents in which a magnesium oxide-forsterite composition is used as the mold surface.
Operhall, U.S. Pat. No. 2,806,271 shows coating a pattern material with a continuous layer of the metal to be cast, backed up with a high heat conductivity metal layer and investing in mold material. Basche, U.S. Pat. No. 4,135,030 shows impregnation of a standard ceramic shell mold with a tungsten compound and firing in a reducing atmosphere such as hydrogen to convert the tungsten compound to metallic tungsten or tungsten oxides. These molds are said to be less reactive to molten titanium but they still have the oxide problems associated with them.
Brown, U.S. Pat. No. 4,057,433 discloses the use of fluorides and oxyfluorides of the metals of Group IIIa and the lanthanide and actinide series of Group IIIb of the Periodic Chart as constituents of the mold surface to minimize reaction with molten titanium. This reference also shows incorporation of metal particles of one or more refractory metal powders as a heat sink material. However, even those procedures have resulted in some alpha case problems. Feagin, U.S. Pat. No. 4,415,673 discloses a zirconia binder which is an aqueous acidic zirconia sol used as a binder for an active refractory including stabilized zirconia oxide thereby causing reaction and gelation of the sols. Solid molds were made for casting depleted uranium. A distinction is made in this patent between xe2x80x9cactivexe2x80x9d refractories and refractories which are relatively inert. The compositions of Feagin are intended to contain at least a portion of active refractories. See also Feagin, U.S. Pat. No.4,504,591.
Some refractory compositions have been developed that exhibit reduced alpha case and can be used successfully to make production castings by applying the coatings to the wax patterns by special techniques, such as spraying. However, a difficulty arises in that certain refractory mixes do not have a long pot life and gel quickly, even spontaneously with stirring in a few minutes, depending upon exact composition. See Holcombe et al., U.S. Pat. No. 4,087,573.
The use of graphite in investment molds has been described in the art in such patents as U.S. Pat. Nos. 3,241,200; 3,243,733; 3,256,574; 3,266,106; 3,296,666 and 3,321,005 all to Lirones. Other prior art which show a carbonaceous mold surface utilizing graphite powders and finely divided inorganic powders called xe2x80x9cstuccosxe2x80x9d are Operhall, U.S. Pat. No. 3,257,692; Zusman et al., U.S. Pat. No. 3,485,288 and Morozov et al., U.S. Pat. No. 3,389,743. These documents describe various ways of obtaining a carbonaceous mold surface by incorporating graphite powders and stuccos, various organic and inorganic binder systems such as colloidal silica, colloidal graphite, synthetic resin which are intended to reduce to carbon during burnout, and carbon coated refractory mold surfaces. These systems were observed to have the disadvantage of the necessity for eliminating oxygen during burnout, a limitation on the mold temperature and a titanium carbon reaction zone formed on the casting surface.
Further developments including variations in foundry molds are shown in Turner et al., U.S. Pat. No. 3,802,902 which uses sodium silicate bonded graphite and/or olivine which was then coated with a relatively non-reactive coating such as alumina. However, this system still did not produce a casting surface free of contamination.
Rammed graphite is used to produce molds for casting of reactive metals and alloys based on titanium. Such molds are made from a mixture of finely divided graphite having a closely controlled particle size and size distribution. Water, pitch, baume syrup and starch are added to coat the graphite powders and provide optimal mold properties.
A number of attempts have been made in the past to coat the graphite and the ceramic molds with materials which would not react with the reactive metals being cast. For example, metallic powders such as tantalum, molybdenum, columbium, tungsten, and also thorium oxide had been used as non-reactive mold surfaces with some type of oxide bond. See Brown, U.S. Pat. Nos. 3,422,880; 3,537,949 and 3,994,346.
Adhesive plasters made of a suspension of oxide powder, such as yttrium oxide and an acid are shown in Holcombe et al., U.S. Pat. No. 4,087,573. These compositions are described as being spontaneously hardening and useful for coating surfaces or for casting into a shape. Of particular interest is the coating of graphite crucible used in uranium melting operations.
Permanent mold casting has been employed in the past as a relative low cost casting technique to mass produce aluminum, copper, and iron based castings having complex, near net shape configurations. However, only fairly recently have attempts been made to produce titanium and titanium alloy castings using the permanent mold casting process. For example, the Mae et al U.S. Pat. No. 5,119,865 issued Jun. 9, 1992, discloses a copper alloy mold assembly for use in the permanent mold, centrifugal casting of titanium and titanium alloys. Mae, et al discloses mold body is made of one alloy selected from a group consisting of a Cuxe2x80x94Zr alloy, a Cuxe2x80x94Crxe2x80x94Zr alloy, a Cuxe2x80x94Be alloy, a Cuxe2x80x94Cr alloy and a Cuxe2x80x94Ag alloy.
Colvin et al U.S. Pat. Nos. 5,287,994 and 5,443,111 discloses metallic permanent mold made of low carbon steel or titanium for fabrication of titanium and nickel based castings. A suitable melt having a relatively low melt superheat is poured into a mold cavity defined by one or more mold members where the melt solidifies to form the desired casting. The melt super-heat is limited so as not to exceed about 150 degree. F above the liquidus temperature of the particular melt being cast. The mold body-to-mold cavity volume ratio is controlled between 10:1 to 0.5:1 to minimize casting surface defects and mold wear/damage. The ""111 Patent discloses the use of a differential pressure is on the melt to be cast so as to assist filling of the mold cavity with the melt. The differential pressure can be established by evacuating the mold cavity relative to the ambient atmosphere while the melt is introduced into the mold. Alternately or in addition, the ambient atmosphere can be pressurized while the melt is introduced into the mold to provide such differential pressure. In still another embodiment of the ""111 Patent, the solidified casting is removed (e.g. ejected) while hot to avoid damage to the casting that could occur as a result of mold constraints associated with a particular complex casting configuration.
Choudhury et al U.S. Pat. Nos. 5,626,179, 5,950,706 discloses a reusable casting mold having a surface which comes in contact with molten metal, the said surface consisting of at least one metal selected from the group consisting of tantalum, tantalum alloys, niobium, niobium alloys, zirconium, and zirconium alloys, and casting in said mold a melt of a reactive metal selected from the group consisting of titanium and titanium alloys.
There is a need for an improved cost effective process for making castings of titanium alloys of various symmetric and asymmetric shapes with thin walls, cylinders, pipe, tubular products and seamless rings with simple or contoured cross sections which can be inexpensively machined into final shapes suitable for jet engine and other high performance engineering applications.
It is an object of the invention to centrifugally cast titanium and titanium based alloys into various complex symmetric and asymmetric shapes as well as tubes, pipes and rings under vacuum or partial pressure of inert gas in reusable isotropic graphite molds, the molds either revolving around its own horizontal or vertical axis or centrifuging around a vertical axis of rotation.
It is another object of the present invention to provide a centrifugal casting apparatus that includes an isotropic graphite mold.
It is another object of the invention to centrifugally cast titanium base alloys in isotropic graphite molds with the mold cavity coated with a thin layer of dense, hard and wear resistant refractory metal carbide and boride coating such hafnium carbide, titanium carbide, hafnium diboride or titanium diboride.
It is another object of the invention to centrifugally cast titanium base alloys in isotropic graphite molds with the mold cavity coated with a thin layer of dense and wear resistant refractory metal coating such tungsten and/or rhenium.
This invention relates to a process for making various metallic alloys such as titanium based alloys as engineering components by vacuum induction or vacuum arc melting of the alloys and subsequent centrifugal casting of the melt under vacuum in isotropic graphite molds, the molds rotating around its own horizontal or vertical axis or centrifuging around a vertical axis of rotation. More particularly, this invention relates to the use of high density high strength isotropic graphite.
With true centrifugal casting, an isotropic graphite metal mold revolves under vacuum at high speeds in a horizontal, vertical or inclined position as the molten metal is being poured. The axis of rotation may be horizontal or inclined at any angle up to the vertical position. Molten metal is poured into the spinning mold cavity and the metal is held against the wall of the mold by centrifugal force. The speed of rotation and metal pouring rate vary with the alloy and size and shape being cast.
As molten alloy is poured into a rotating isotropic graphite mold, it is accelerated to mold speed. Centrifugal force causes the metal to spread over and cover the mold surface. Continued pouring of the molten metal increases the thickness to the intended cast dimensions. Rotational speeds vary but sometimes reach more than 150 times the force of gravity on the outside surface of the castings. Once the metal is distributed over the mold surface, solidification begins immediately. Metal feeds the solid-liquid interface as it progresses toward the bore. This, combined with the centrifugal pressure being applied, results in a sound, dense structure across the wall with impurities generally being confined near the inside surface. The inside layer of the solidified part can be removed by boring if an internal machined surface is required.
For specialized engineered shapes, centrifugal casting offers the following distinct benefits of titanium based alloys:
(1) Any titanium common to static pouring under vacuum can be centrifugally cast in accordance with the present invention as a tubular product, ring and pipe.
(2) Mechanical properties of centrifugally cast titanium according to the present invention will be excellent.
Centrifugal castings of titanium base alloys can be made in almost any required length, thickness and diameter. Because the mold forms only the outside surface and length, castings of many different wall thicknesses can be produced from the same size mold. The centrifugal force of this process keeps the casting hollow, eliminating the need for cores.
Horizontal centrifugal casting technique is suitable for the production of titanium alloys pipe and tubing of long lengths. The length and outside diameter are fixed by the mold cavity dimensions while the inside diameter is determined by the amount of molten metal poured into the mold.
Castings other than cylinders and tubes also can be produced in vertical casting machines. Castings such as controllable pitch propeller hubs, for example, can be made using this variation of the centrifugal casting process.
The outside surface of the casting or the mold surface proper can be modified from the true circular shape by the introduction of flanges or small bosses, but they must be generally symmetrical about the axis to maintain balance. The inside surface of a true centrifugal casting is always cylindrical. In semi-centrifugal casting, a central core is used to allow for shapes other than a true cylinder to be produced on the inside surface of the casting.
The uniformity and density of centrifugal castings approaches that of wrought material, with the added advantage that the mechanical properties are nearly equal in all directions. Most alloys can be cast successfully by the centrifugal process, once the fundamentals have been mastered. Since no gates and risers are used, the yield or ratio of casting weight-to-weight of metal is high. High tangential strength and ductility will make centrifugally cast titanium alloys well-suited for torque- and pressure-resistant components, such as gears, engine bearings for aircraft, wheel bearings, couplings, rotor spacers, sealed discs and cases, flanges, pressure vessels and valve bodies. Titanium alloy melts do not react with high density, ultra fine grained isotropic graphite molds and hence, the molds can be used repeatedly many times thereby reducing significantly the cost of fabrication of centrifugally cast titanium alloy components compared to traditional processes. Near net shape parts can be cast, eliminating subsequent operating steps such as machining.
A motor may be employed for spinning the centrifugal casting apparatus according to the present invention. In one embodiment, the mold may be two longitudinally split pieces. In another embodiment, the mold may be two transversely split pieces.
In the centrifuge casting of titanium in isotropic graphite molds, the molds may be located on the circumference of a horizontal circle. Mold cavities are connected via radial runner-gate assembly to a central downsprue located along the vertical axis at the center of the circle.
Simultaneous rotation of a tree of molds located along the perimeter of circle on a horizontal plane while melt is being poured into a central downsprue lying along the vertical axis of the tree creates high velocity flow of melt under the action of centrifugal force. Melt is forced through the runner into the mold cavities filling thin sections with attendant fine detail and form. The centrifugal force allows the melt to flow into even the most obscure crevices of the mold cavities. Centrifugation is maintained until the melt solidifies.
The centrifugal force imposed on the melt enhances removal of gas bubbles and reduces the number of gaseous defects to a minimum and improves the mechanical properties of the castings. An additional advantage of centrifugation is a more efficient use of metal due to the parabolic free surface of the liquid metal in the mold. The metal charge weight can be carefully adjusted for each mold configuration to ensure the filling of each casting cavity and its xe2x80x98runnerxe2x80x99, while leaving a significant portion of the central down sprue devoid of metal.
The titanium castings that can be produced in accordance with the scope of the present invention will find many diverse applications, for example aero engine components and airframe structural parts, missile guidance components requinng a coefficient of expansion very similar to glass, high strength cryogenic parts for space exploration, and fatigue resistant and tissue compatible surgical implants.
In accordance with the present invention based on centrifugal casting of titanium alloys, the durability of the high density high strength isotropic graphite molds can be further enhanced by having the mold cavity coated with a hard wear resistant coating of refractory metal carbide such as hafnium carbide or refractory metals such as tungsten or rhenium. Such coatings with desirable properties and thickness between 2 to 200 microns and preferably 10-25 microns can be produced on the machined cavity of the isotropic graphite mold via one of the processes such as the chemical vapor deposition (CVD), sputtering, magnetron-sputtering or plasma assisted chemical vapor deposition techniques.
The present invention has a number of advantages:
(1) Use of ultrafine grained isotropic graphite molds to fabricate titanium castings improves quality and achieves superior mechanical properties compared to castings made by a conventional investment casting process.
(2) The molds can be used repeatedly many times thereby reducing significantly the cost of fabrication of castings compared to traditional process.
(3) Near net shape parts can be cast, eliminating subsequent operating steps such as machining.
(4) The castings can be made in molds held at room or low temperatures resulting in finer grain structures and improved mechanical properties.